1. Field of the Invention
The invention in general relates to steam turbines, and particularly to a system for maintaining proper temperature in the gland sealing system of the turbine.
2. Description of the Prior Art
A steam turbine ordinarily has a shaft, or rotor, resting in bearings and enclosed in one or more casings, referred to as cylinders. At the point where the rotor penetrates the outer cylinders some means is required in order to prevent leakage of air into, or steam from, the cylinders. Members known as glands having labyrinth-type seal rings in conjunction with a gland sealing steam system are provided to perform this function.
During startup or at relatively low load conditions, sealing steam for the glands is provided by a steam supply such as an auxiliary boiler designed for this purpose. Once the turbine is running at higher load levels, the steam for sealing the gland is provided from within the turbine itself such as by exhaust steam, during which condition the system is self-sealing.
Some turbines are designed such that the turbine inlet steam is utilized to self-seal a gland, in which case the steam temperature for sealing is much higher than that provided by an auxiliary system. If the turbine is suddenly tripped, or if the load drops below a predetermined level, sealing switches from self-seal back to the auxiliary system at the much lower temperature. This subjects the rotor to an objectionable thermal shock due to the difference in temperatures between the sealing steam, and thus reduces the life of the rotor. Conversely, during startup conditions sealing steam will switch from the relatively low temperature auxiliary to the relatively higher temperature inlet steam again subjecting the rotor to the objectionable thermal shock.
The present invention provides for an improved gland sealing system which minimizes or eliminates the objectionable thermal shock and therefore increases rotor life.